It is generally known with such processes to take measures so that the reaction components arrive at the mixing operation in the conditioned state and also conditioning the line system itself, so that defect-free moldings can be produced. A constant temperature of the reaction components and of the system from the beginning through the entire operating time and having constant and reproducible pressures immediately before and during the shot time are important for the conditioning.
When there are volume changes in the line system due to changes in pressure during operation, these are referred to "breathing" of the system. However, volume also depends on its thermal expansion. Maintaining the temperature conditions presents substantial problems over long idle or even standstill times. For the production of a molding with lesser requirements, a gear pump or a spiral pump is used as the metering unit. To produce moldings of higher precision, it is essential to use a high-speed metering reciprocating pump, for example, a series reciprocating pump or an axial or radial reciprocating pump. For viscous and filler-containing reaction components, reciprocating metering apparatuses are used. Mixing heads with ejection pistons are used, in particular, as the mixing unit. According to such a process, the production process is conventionally controlled semi-automatically or completely automatically wherein a central control apparatus of the device acts on the pumps, such as pre-pressure pumps, feed pumps, metering pumps or optionally, reciprocating metering apparatuses, as well as on the valves, such as shut-off valves, optionally, throttle valves and reversing valves, and optionally, on the control pistons or the jet needles of a mixing unit. Such a device is, as a rule, the metering and mixing component of an installation, to which a component carrying several molds, for example a circular table, also belongs. A central control apparatus can also be assigned to the entire installation, or a second control apparatus coupled to the first control apparatus is provided for the component carrying the molds. (See, generally, Kunststoff Handbuch [Plastics Handbook], volume 7"Polyurethane [Polyurethanes]", Carl Hanser-Verlag Munich/Nienna, 1983, pages 121 to 168).
Therefore, conditioning of the metering pump has not been taken into consideration when conditioning the device.
As a general rule, the metering pump has an enclosed metering component, such as, for example, an axial reciprocating metering pump with several pistons and cylinders. This enclosed metering component is surrounded by a pump housing, wherein an intermediate space between the housing and metering component is present to collect liquid, which has leaked out.
It has been found that the metering pump, which is of stable construction per se, or its metering component, is likewise subjected to the influence of temperature. Although the temperature-controlled reaction component has an effect on the metering component in terms of temperature from the inside during metering, towards the outside there is a temperature gradient which depends on the ambient temperature, which varies. The intrinsic heating of the pump also has an adverse effect in this connection.